A stacked piezoelectric element in which piezoelectric ceramic layers formed of a PZT-based material or the like having excellent piezoelectric properties and dielectric properties and electrode layers formed of a base metal such as copper are alternately stacked, is being used for an actuator, a capacitor and the like. The method for producing such a stacked piezoelectric element usually comprises the following multiple steps.
A green sheet formed of a ceramic material such as PZT is prepared, and an electrode paste material is coated on this green sheet by screen printing or the like. Subsequently, a desired number of green sheets each coated with the electrode paste material are stacked to produce a stack, and this stack is degreased. The degreased stack is heated in a heating furnace under a reducing condition to reduce the metal oxide in the electrode paste material and form an electrode layer having electrical conductivity (hereinafter referred to as an “electrode reducing step”). Thereafter, this stack is fired to be densified the ceramic material to produce a final stacked piezoelectric material (hereinafter referred to as a “firing step”).
In order to obtain a good joined state at the boundary and high durability of the joined state, the ceramic material and the electrode paste material are preferably fired at the same time. However, in the firing step, the electrode paste material and the ceramic material require contradictory atmosphere conditions. More specifically, for example, the ceramic material such as PZT is an oxide and is preferably fired in an oxidizing atmosphere, whereas in order to maintain the electrical conductivity obtained in the electrode reducing step, the electrode paste material is preferably fired in a reducing atmosphere.
In the firing step, when the firing is performed in an oxidizing atmosphere so as to satisfactorily fire the ceramic material, the electrode layer formed of copper or the like, which is reduced in the previous electrode reducing step, is sometimes again oxidized to decrease the electrical conductivity. On the other hand, when the firing is performed in a reducing atmosphere, the ceramic material is reduced to cause a problem that the characteristics of the stack after firing are impaired. The “oxidizing atmosphere” as used herein means an atmosphere condition where, in the stacked product manufactured from a ceramic material and an electrode metal and the atmosphere of the firing step is shifted to the relatively oxidizing side as compared with the peripheral condition allowing for the metal state of the electrode metal and for the-oxide state of the ceramic material. The reducing atmosphere means similarly an atmosphere condition where the atmosphere of the firing step is shifted to the relatively reducing side.
In order to solve the above-described problem, Japanese Unexamined Patent Publication (Kokai) No. 5-82387 describes a method of reducing an electrode paste material in an atmosphere containing a hydrogen gas and, in the subsequent firing step, firing the stack in an atmosphere controlled to an oxygen partial pressure in a specific range.
Also, Japanese Examined Patent Publication (Kokoku) No. 7-34417 describes a method of reducing an electrode paste material at a temperature lower than the firing temperature and in the subsequent firing step and firing the stack by using an N2—H2—H2O—O2 mixed gas in an atmosphere controlled to an O2 partial pressure in a specific range.
According to these conventional methods, the ceramic material can be densified in the firing step almost without oxidizing the electrode layer comprising copper or the like, which is reduced in the electrode reducing step. In addition, there is a known method comprising a step of printing an electrode paste material mainly comprising an electrode metal, which is constituted to contain no oxide or have a small oxide content; a degreasing step of performing degreasing while preventing oxidation of the electrode metal in the electrode paste material by controlling the oxygen partial pressure; and a firing step of densifying the ceramic material.
However, these conventional methods have the problems that, in the case of producing a stack having a large number of layers stacked, the degree of reduction•oxidation reaction can be hardly balanced between the center part and the outer peripheral part of the stack and the adjustment of the atmosphere is very difficult. Also, when the stack has a large shape, an enormous amount of time is required for the removal of binder in the degreasing step before firing or before firing•reduction of the electrode. Furthermore, when a part of the binder remains, the atmosphere in the after step cannot remain uniform due to oxidation of the remaining substance and this adversely affects the quality of the piezoelectric element produced.
PZT, which has been used in recent years, contains Pb as a component element thereof. The oxide of Pb sublimates or liquefies at around 900° C. and then exits from the medium. As a result, the composition changes to cause relative deterioration in the performance as a piezoelectric material. In order to overcome this problem, during production, the amount of Pb-based material, as a starting raw material, is increased in advance, or at the firing of the piezoelectric element, Pb or a Pb feed material is disposed in the periphery of the piezoelectric element, so that a Pb atmosphere can be formed and the composition can be prevented from changing. In some cases, the property peculiar to the oxide of Pb is used the Pb containing material is increased, or another material is added to the oxide of Pb, so that a liquid phase of PbO (or a eutectic substance of PbO and the another material) can be formed to elevate the transmittance of heat and in turn accelerate the sintering, thereby achieving low-temperature sintering.
The PZT material produced by using an oxide of Pb has been aggressively used because the piezoelectric performance is particularly excellent or firing can be performed at a relatively low temperature to allow for broadened selection of the electrode material, though this material is difficult to use when the quality of the product produced is taken into account.
However, due to environmental problems and as Pb is a harmful substance, a piezoelectric material not containing Pb as the constituent element is being demanded. Also, for producing a piezoelectric element at a low cost, the electrode material must be a base metal and not a noble metal. In this case, as described above, both the oxidized state of ceramic material and the reduced state of electrode layer must be established at the same time.
In the case of a Pb-containing piezoelectric material, the firing temperature can be low and therefore, Cu having a relatively low melting point, but not Ni having a high melting point, can be used. Furthermore, as Cu tends to be less ionized as compared with Ni and has a high free Gibbs energy for oxidation, the oxidized amount is small even under a relatively high oxygen partial pressure (about 10−6 atm) and therefore, Cu can be maintained as an electrically conducting metal by adjusting the reducing atmosphere.
The degree of difficulty in maintaining a metal as an electrically conducting metal can be determined as follows. For example, the impurity determined of a cylinder gas is usually on the order of 0.01 to 1%. For example, in the case of Cu (about 10−6 atm), even if a slight amount of oxygen is contained therein, the oxygen amount is not at a level to greatly inhibit the adjustment of oxygen partial pressure (the level is equal to or lower than, for example, error in the change of temperature, humidity or the like or in the change of material composition, and the contribution to the degree of oxidation of the electrode material or ceramic material is small).
On the other hand, as for the Pb-free material, studies are at present devoted exclusively to elevate the piezoelectric performance, and two approaches remain as matters to be studied. That is, one approach is to elevate the piezoelectric performance while adding a material of forming a liquid phase due to low-temperature firing and another approach is to perform the firing by employing an electrically conducting base metal material having a high melting point as the electrode material.
For example, as for the electrode metal having a high melting point, Ni is representative of the material usable even at a relatively high temperature (from 1,100° C. to 1,300° C.). Ni is readily oxidized as compared with Cu and therefore, the oxygen partial pressure must be controlled to be lower. However, as described above, the gas for adjusting the atmosphere contain impurities and in fact, as the oxygen partial pressure becomes lower, the noise ascribable to those impurities relatively increases. As a result, although the metal state of electrode layer and the oxide state of ceramic layer are created in some samples, these desired states are not necessarily created in other samples and a poor-quality product results. Also, when it is intended to perform low-temperature firing by using a Pb-free ceramic material, the atmosphere adjustment becomes more difficult when combined with restriction of the oxidized state of, for example, an additive used for the low-temperature firing. Accordingly, in either study, difficulty in adjustment of the atmosphere occurs.
One example of the deterioration in quality of a product when the electrode layer and the ceramic layer are not in respective desired states described above is briefly described below.
When the atmosphere is shifted to the oxidizing side of the optimal atmosphere condition at the time of forming a reducing atmosphere and, for example, at the time of firing, the electrode may be oxidized and, by causing a eutectic reaction with the ceramic layer comprising an oxide material, melted and diffused. Also, when the atmosphere is shifted to the reducing side of the optimal atmosphere, a part of the ceramic layer is reduced and sometimes causes a eutectic reaction with the metal of the electrode layer, giving rise to melting and diffusion of the electrode material. A eutectic reaction readily occurs between a metal and a metal or between an oxide and an oxide. In the case of a combination of metal and oxide, occurrence of a eutectic reaction is extremely rare.
In this way, when the electrode layer and the ceramic layer are not in the respective desired states, the material of an electrode layer and the material of a ceramic layer undertake a eutectic reaction to cause disconnection of the electrode layer in the middle and, as a result, the electrode layer can only partially function and the performance decreases. Other than this, various other problems are possible, such as decrease in the electrical conductivity due to oxidation of the electrode layer, decrease in the insulating resistance due to reduction of the ceramic layer, and loss of piezoelectric property in a part of the ceramic layer material.
At the production of a piezoelectric material, starting raw materials are mixed to give a desired ratio of constituent atoms, but all materials are not consumed to constitute the piezoelectric material compound and there occurs a phenomenon that a small portion of the material fails to encounter a constituent material with which the material should be compounded, and remains without constituting the piezoelectric material compound. This is a well-known phenomenon. As described above, oxides or metals readily undertake eutectic formation•melting with each other, but eutectic formation hardly occurs between an oxide and a metal. Therefore, the firing must be performed without oxidizing the electrically conducting base metal material and at the same time, without reducing the piezoelectric material and the remaining substances. To satisfy this requirement, setting of the atmosphere conditions becomes very difficult. When a material containing Pb, particularly a Pb feed material, is used, sublimation or liquefaction of Pb takes place at the firing and the material readily remains without forming a compound having piezoelectricity.
These problems encountered in the adjustment of atmosphere are greatly affected by the impurities at the atmosphere adjustment and readily arise in the case of a Pb-free material, but the Pb-containing material is not completely free of these problems. Also in the case of a Pb-containing material, the problems encountered in the adjustment of atmosphere may be solved by controlling various conditions necessary for the formation of atmosphere, such as hydrogen partial pressure, but various conditions for the formation of an atmosphere cannot be easily controlled.
Due to difficulty in view of production method, as described above, the electrode material not having piezoelectric performance may flow into the ceramic layer having piezoelectric performance during firing and segregate there, and this causes a problem in the quality of the finished product. In an element where a relatively large material block is segregated, displacement is not caused in the segregated material at the time of applying a voltage to displace the piezoelectric layer, as a result, stress is concentrated on the boundary with the segregated material and, depending on the case, there arises a problem in durability, such as the generation of cracks.
In order to solve these problems, Kokai No. 2002-260951 describes a technique of adding a melting suppressing substance or a melting point elevating substance to an electrode paste material comprising an organic vehicle and an oxide of an electrically conducting base metal material or comprising an organic vehicle, an electrically conducting base metal material and an oxide thereof. As a result, an effect that, even when the electrically conducting base metal electrode is oxidized after firing, the electrically conducting base metal material is not segregated in the ceramic layer is obtained. If the electrode material is not segregated even when oxidized, the difficulty in controlling the atmosphere, which is a common problem in Ni, Cu and/or a compound or mixture thereof as described above, can be overcome. In Kokai No. 2002-260951, the electrically conducting base metal material can be prevented from segregation in the ceramic layer by the above-described addition of a melting suppressing substance or a melting point elevating substance, but it is not indicated whether the added melting suppressing substance or a melting point elevating substance itself is satisfactorily dispersed in the layer.
Furthermore, the added melting suppressing substance or a melting point elevating substance itself has no piezoelectric performance and therefore, similarly to the electrically conducting base metal material, if such a substance is localized in the ceramic layer, a stress is concentrated thereon and this gives rise to generation of cracks. Also, if such a substance is localized in the electrode layer, the electrical conductivity of the electrode is impaired and the piezoelectric performance decreases.
Insofar as a Pb-containing material is used, the problems ascribable to the sublimation or liquefaction peculiar to Pb remain, and instability of quality is still unresolved. As a result, it is considered that, at the time of firing a large piezoelectric element, the Pb content becomes non-uniform in the front, rear, left, right, top and bottom of the element and good performance cannot be fully exerted.
The present invention has been made to solve these problems in conventional techniques.
An object of the present invention is to provide a stacked piezoelectric element with excellent durability, which exhibits an excellent piezoelectric performance irrespective of containing or not containing Pb in the ceramic layer, ensures sufficiently suppressed segregation of the electrically conducting base metal material in the ceramic layer and allows for no segregation of a strengthening substance itself, which is used to suppress the segregation of the base metal electrode material; and an electrode paste material used for the production.
Another object of the present invention is to provide a Pb-free stacked piezoelectric element and a production method thereof. In particular, the present invention provides a method for producing a ceramic product with excellent quality by suppressing the segregation of the electrically conducting base metal material in the ceramic layer, which is caused due to difficulty in the adjustment of atmosphere.